143B
c. A. M U C C I N I AND ROBERT13. SCHULER
\rol. ( i l
RADIATIOK CHEMISTRY OF CYCLOPENTANE-CYCLOHEXAXE MIXTURES’ BY G. A. MUCCINI AND ROBERT H.SCHULER fifellonInstitute, Radiation Research Laboratories, Pittsburgh, Pennsylvania Received March $1.1.960
Radiolysis of mixtures of cyclopentane and cyclohexane with -prays a t 70,000 rads/hr. results in a greater yield of products derived from cyclopentane than would be expected from the fraction of this component in the system. This effect is attributed to a change in the identity of the intermediate radicals as a result of preferential abstraction of hydrogen from the cyclopentane. Studies of the primary radical yields by iodine scavenging methods and of the effect of intenaty on the products from the mixed hydrocarbons confirm this explanation.
Previous investigations of the radiation chemistry of mixed hydrocarbons have focussed attention on the non-ideal behavior of systems where energy transfer between the various components is possible. Such non-ideality is particularly emphasized in the early work of Manion and Burton where the yields of gaseous products from benzene-cyclohexane solutions were found to be less than expected from the composition of the system.* The present work was undertaken in order to examine a system (cyclopentane-cyclohexane) in which the radiation chemical effect on each of the components present might well be expected to be proportional to their concentration. As is seen below, even when the inital chemical products do correspond to such an expectation, changes in the identity of the intermediate radicals due to purely chemical processes can result in a departure of the over-all behavior a t low absorbed dose rates. As previously reported3 approximate ideality returns a t high dose rates where secondary chemical processes interfere less with the ultimate determination of the products.
Experimental Phillips Research Grade hydrocarbons were used in all investigations. An olefinic impurity in the cyclohexane, which was detected by the brownish color of its iodine solution, was removed by passing the hydrocarbon through a silica gel adsorption column. Samples were outgassed and sealed before irradiation. Irradiations were carried out for the most part a t room temperature in ti Brookhaven type cobalt-60 source a t an absorbed dose rate of 70,000 rads/hr. Absolute yields have been calculated relative to the yield of the Fricke dosimeter [G(Fe+++)= 15 51 with the assumption that energy absorption is proportional to the electron density of the irradiated sample. Certain additional irradiations were carried out a t other dose rates where only the relative yields of products were measured. The total absorbed dose was approvimately 3 X lozoe.v./g. ( 5 megarads) except in experiments in which 0.003 M iodine was employed to measure the initial yield. In these scavenger studies the absorbed dose was limited to 6 X 101pe.v./g.in order to maintain a significant iodine concentration throughout the irradiation. After irradiaticm the samples were examined gas chromatographicallv with attention being focussed on the products in the (2x0 to (212region. Fractionation was on a250 em. column of 25% silicone grease on firebrick and detection was by thermal cc~nductivitymethods. Typical chromatograms are shown in Fig. 1 . The major products are cyrlopentylcyclopentane, cyclopentylcyclohexane and cyclohexylcyclohexane. Small amounts of other products complicate the observed vegion to a certain extent but do not appear to alter the main conclusions drawn below. The sensitivity of (1) Supported, in part, by the U. S. Atoniic Energy Commission. (2) J. Mmion and M. Burton, THISJOURNAL, 66, 560 (1052). (3) R. I3 SrLiiler and G. A . Muccini, J . A m . Chem. SOC.,81, 4115 (Io.59).
the gas chromatographic apparatus was established with solutions containing known amounts of cyclopentyl and cyclohexyl iodides and of cyclopentylcyclopentane and cyclohexylcyclohexane. Since almost identical sensitivities were observed for the last two substances it was assumed that a mean calibration would hold for cyclopentcyclohexane. Only these three major products are considered below.
Results and Discussion In the following discussion it is assumed that absorption of energy by each component of a mixture is proportional to the electron fraction present. This assumption is in turn based on the premise that the cross sections for the interactions of the ionizing electrons are independent of the nature of the bound electrons. Since radiation chemical effects are to a large extent ultimately produced by secondary and tertiary electrons having very low energies the above assumptions are obviously not generally valid. I n particular these assumptions are not true for mixtures containing heavy atoms where the inner very energetically bound electrons contribute only slightly to the absorption. Mixtures of cyclopentane and cyclohexane however, provide an example where these assumptions must be very nearly valid. Since the empirical formulas of the components are the same, the average cross sections for electron-electron interactions must be identical (except for very slight differences due to chemical binding). Barring energy transfer between the components the initial chemical decomposition of each of the components should be proportional to the electron fraction present. In Fig. 2 the yields are given for the various “dimer” products formed in the y-ray experiments a t 70,000 rads/hr. It is seen that the curves are skewed somewhat in favor of products derived from cyclopentane in spite of the fact that the yield of cyclopentylcyclopentane from pure cyclopentane is somewhat less than the corresponding yield from cyclohexane. This effect is readily seen in the upper chromatographic curve of Fig. 1,where for solutions 50 electron % in each component the ratio of yields of cyclopentylcyclopentane to cyclopentylcyclohexane to cyclohexylcyclohexane is 1.65: 2.00 :0.72. If each component were affectedto the same extent a ratio of 1 :2 : 1 would, of course, be expected. If the yield of “dimer” from the pure components represents the decomposition, then a ratio of 0.75 : 2.00:1.25 would be expected. It is seen that this latter ratio is nearly identical to the value of 0.70: 2.04: 1.26 observed a t high inten~ities.~ The favoring of products derived from cyclopentane is particularly obvious for solutions dilute in cyclo-
RADIATION CHEMISTRY OF CYCLOPENTANE-CYCLOHEXANE R~IIXTURES
Oct., 1960
20
0
Go6' y-rays ( 7 x IO4 rads/h)
E. 3
105
Fig. 2.-Yield of dimer products as a function of solution composition: 0, cyclopentylcyclopentane; A, cyclopentylcyclohexane; 0, cyclohexylcyclohexane.
I
Co60 y-rays ( I x !Osrods/h)
40 60 E l e c t r o n Per C e n t CycloPexane.
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1
0
40
60
80
I
Electron Per Cent Cyclohexane.
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Time Fig. 1.-Gas chromatograms of products in the Clo-C12 region from cyclopentane-cyclohexane solutions irradiated at various intensities. Solutions are 50 electron yo in each component.
pentane, e.g., for solutions containing only 10% the total contribution of cyclopentane to the "dimer" products is 3.5 times the expected value. These studies were extended by two types of experiments to determine if the above observations were a manifestation of the importance of physical processes such as transfer of energy from one component to the other or rather due to a purely chemical effect involving the preferential abstraction of hydrogen by the cyclohexyl radicals formed in the radiolysis, %.e.,ICl > kz for the reactions CGtjLI' 4- C5HlO ----f C6HlZ C ~ t € s3 . C ~ H I+ Z C5HIO
20
+ COIIQ' + C~HII.
(1) (2)
First the effect of intensity was examined and as previously reported3 showed that the contribution of cyclopentane to the decomposition decreases significant1.y as the intensity increases. This is readily explaincd in terms of a decreased lifetime of
Fig. 3.--Iodide yields produced in the scavenging of cyclopentane-cyclohexane solutions with 0.003 M iodine: 0 , cyclopentyl iodide; 0, cyclohexyl iodide; A, cyclopentyl iodide cyclohexyl iodide.
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the intermediate radicals due to the increased rate of the combination reaction R * R.'+RR' (3) relative to 1 and 2 . If it is assumed that the bimolecular rate constant for reaction 3 is less than 10" liter mole-' s a - ' then it can be shown that for the intensities at which the present y-ray experiments are carried out the lifetime of the intermediate radicals will be greater than 10 mseconds. Abstraction reactions which occur with reasonably low activation energies (less than 8 kcal./mole if we neglect steric and diffusion terms) should then effectively compete with 3. If, as was originally expected, the rate constants for reactions 1 and 2 were the same then no change in composition would result from the abstraction reactions. Apparently, however, abstraction of hydrogen from cyclopentane occurs somewhat more readily than from cyclohexane. Since these reactions probably recur a number of times before the ultimate determination of products it is necessary only that the rate coii-
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AVROMA. BLUMBERG AND STAVROS G. STAVRINOU
1438
Yol. 64
ratio to increase with intensity (to 2.2 a t 7 X lo8 rads/hr.) was observed and may be due to underlying reactions which produce “dimer” product by other processes. The second type of experiment involved the use of iodine to scavenge the free radicals produced initially. Unfortunately because of the sensitivity limits presently imposed by chromatographic methods such studies must be carried out a t relatively high iodine concentrations (-0.003 M ) and are accordingly plagued by unresolved questions concerning the use of scavengers a t these concentration~.~ The results, however, appear to be in complete agreement with the conclusions reached above. It is seen in Fig. 3 that the yields found for the inCEH9 C i ” --+ CIOHI~ (4) dividual radicals in the scavenging experiments are CsH9. CsHii, --+ CnHzo (5) directly proportional to the electron fraction of the CsHii, CaIIii. --+ CizHzz (6) l f combination is purely statistical, ie., k4:kS: ks = parent material present. This is expected since 1:2 : 1 then the product ratio [CIIHzo]/[CloH~sl’/~secondary chemical processes should not interfere [C13H22:1~2 should be equal to 2. A ratio as they do in the absence of scavenger. Energy between 1.8 and 1.9 was found for all the transfer between cyclopentane and cyclohexane apexperiments of Fig. 2. This shows that parently does not occur and we observe ideal bedifferentiation does not occur as a result havior. This result also substantiates the assumpof a favored combination reaction, a possibility tions on energy absorption made in the initial parasince a fraction of the radicals are lost in dispro- graph of the discussion. portionation processes. A slight tendency of this (4) R. H. Sohuler, THISJOURXAL, 61, 1472 (1957).
stants of 1 and 2 be very slightly different in order to explain the present observations. Change in the identity of radicals a t the low intensities of most y-ray experiments can be expected to be even more pronounced in cases where easily abstractable hydrogen atoms are present. The change of product composition with intensity shows that chemical processes other than those of first order are involved. This rules out reactions of ions or excited species with solvent molecules as the sole source of these “dimer” products. Presumably these products result to a considerable extent from the combination of cyclopentyl and cyclohexyl r:idicals according to the competing reactions
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TABXJLA’I’ED FUKCTIOSS FOR HETEROGENEOUS REACTIOS liATES : THE ATTACK OF VITREOUS SILICA BY HYDROFLUORIC ACID BY AVROMA. BLUMBERG’ AND STAVROS C. STAVRINOU] Mellon Institute, Pittsburgh 13, Pennsylvania Received March 81 1960 ~
The ra,te equ3tion describing the reaction between a solid and liquid has been integrated in tabular form for the cases where the partial order with respect to the solute (liquid) reactant, n = 1/2, 1, S/Z and 2, and has also been integrated in functional form for 15 = 1 . The reaction between powdered vitreous silica and aqueous hydrofluoric acid in strongly acidic media is very closely first order with respect to hydrofluoric acid concentration, and the order and rate constant obtained here agree with those calculated from the results of a different method.
Introduction The reaction between hydrofluoric acid and silica was prlobably discovered by Scheele a t the time he first prepared the acid in 1771.2 Berzelius jdent’ified the products as silicon tetrafluoride and water,3 and later the combination of the tetrafluoride with hydrofluoric acid to form the stable fluorosilicic acid was ~ b s e r v e d . ~This last is a strong acid, comparable to sulfuric acid, and does not etch glass or attack silica. Early studies on the rate of attack include the work of Gnutiers who compared the degree of attack on glass, fused silica and two faces of crystalline quartz; of Lebrun6 who measured the velocities of dissolutdon of quartz, along four different faces; (1) Pittsburgh Plate Glass Company Research Project. (2) A. :B. Burg, “Fluorine Chemistry,” Vol. I (J. H. Simons, Mitor). Academic Press, Ino.. New York, N. Y.. 1950, p. 180. ( 3 ) J. J. Berselrus, Pogg. Ann., I, 169 (1824). (4) N. V. Sidmvick, “The Chemical Elements and their Compounds,” Vol. 1, (Oxford University Press, Oxford, England, 1950, p. 615. ( 5 ) 4 . IGautier and P. Clausmann, Compf. rend., 167, 176 (1913). ( 6 ) J. Lebrun, R d 1 . Classe Sci. Aeod. Roy. Belg., 953 (1913).
and of Schwarz? who exposed, in turn, quartz, tridymite, cristobalite and amorphous (vitreous) silica to hydrofluoric acid and found the amount dissolved to increase in the order given. More recently, Palmers found the rate of attack of “Vitreosil” to be related not to hydrofluoric acid concentration but to bifluoride concentration. At low ionic strength the rate increased with this property, too. Hydrogen ion had a catalytic effect. Nevertheless, reaction has been observed using hydrogen fluoride gasgand in some aqueous systems of high acidity,s in neither case of which is there much bifluoride ion. It seems worthwhile to resolve this point by studying systems in which conipeting reactions are ruled out; and t,he present study, limiting the attacking species t o hydrofluoric acid alone, was undertaken. (7) R. Schwarz, 2. onorg. Chem., 7 6 , 422 (1912). ( 8 ) W. G. Palmer, J. Chcm. Soc., 1656 (1930). (9) W. K. Van Hagen and E. F. Smith, J. A W k . Chen. Yoc., SS, 1504 (1911).
